* Declarative User Interface: GWT's UiBinder now allows you to create
user interfaces mostly declaratively. Previously, widgets had to be
created and assembled programmatically, requiring lots of code. Now,
you can use XML to declare your UI, making the code more readable,
easier to maintain, and faster to develop. The Mail sample has been
updated to use the new declarative UI.

This forum needs your help: you got hints from the community and now you have fixed your code? dont just reply with "now its fixed" or "i found the error"! please take the time to post also an detailed answer with the working code.

I have just started playing with the new GXT 2.1.1 for GWT 2.0 and I was able to use UIBinder and GXT widgets together.
I noticed that in order to define widgets in a GXT Container instead of a GWT Panel all that is needed is that Container implements the interface HasWidgets. I tried to extend a ContentPanel to implement the HasWidgets interface, but it wasn't possible only because of one difference: method add() returns boolean instead of void.
I would be great if this change could be made to Container so we can use all of GXT's container classes in the UIBinder xml.